Low back injuries continue to be the leading cause of musculoskeletal disorders (MSDs) in the workplace and, according to the most recent U.S. Bureau of Labor Statistics survey, they account for approximately 38% of all workplace MSDs. Because many low back injuries are a result of manual material handling (MMH) activities, various MMH assessment tools have been developed to quantify low back risk, such as:
- NIOSH Lifting Equation for two-handed lifting and lowering
- Liberty Mutual tables for two-handed lifting/lowering, carrying, and pushing/pulling of carts
- BWC/OSU One-Handed Lifting Guidelines
An important distinction regarding these assessment tools is that they only quantify risk for MMH activities performed by a single individual. But how do we assess risk for two-person or team lifts? This is a question that clients often ask us, which can be difficult to answer since there are multiple research studies, recommendations, and guidelines regarding team lifts. However, there are two guidelines that stand out because they are more comprehensive than the rest: Military Standard 1472G (MIL-STD-1472G) and the Health and Safety Executive manual handling assessment charts (HSE-MAC).
First, let’s take a look at the maximum weight limits for infrequent MMH activities (lifts and carries that occur once per hour to once per day), summarized in the table below:
You may notice that, as more people are added to the team lift, the increase in the maximum weight limit isn’t linear when compared to the 1-person lift weight limit. For example, if the increase were linear, the MIL-STD-1472G weight limit for a 4-person lift should be 176 lb (44 lb x 4 people), but it’s not. This is also true for the low-risk and high-risk limits of HSE-MAC. Why is that? There are various factors that contribute to this non-linear increase, one of them being uneven weight distribution during the lift or carry. Often, weight of the object being handled is not evenly distributed, and poor coordination of team lifts may result in one team member supporting more weight than the rest. The question is, how much more?
Based on team lifting studies and guidelines, we can make approximations on how much weight one team member may be supporting at a certain point in time:
For example, if 3 people are performing a team lift with an object that weighs 150 lb, one team member may be supporting about 60 lb of that weight (150 lb x 0.4). You can then enter this weight data into the NIOSH Lifting Equation or the Liberty Mutual carry analysis to arrive at a rough estimate for low back risk, particularly for more repetitive MMH activities. But keep in mind that the NIOSH Lifting Equation and Liberty Mutual tables were developed for a single individual performing the task and are not validated for team lifts. Although you can get an estimate of how acceptable a load might be by using these adjusted weights, you can’t conclude that a low risk finding indicates zero risk of injury.
Finally, it’s important to recognize that administrative controls, such as team lifts, are not very effective in reducing MSD risk and should only be used as short-term solutions. Long-term engineering solutions, such as lifting devices, should be implemented to permanently reduce MSD risk in the workplace.
For more on this topic, read the blog “A How-To Guide: The NIOSH Lifting Equation.”
References
Bureau of Labor Statistics. (2018). Survey of Occupational Injuries and Illnesses Data: MSD by part of by affected by days away from work.
https://www.bls.gov/iif/soii-data.htm#newsrelease?utm_source=rss&utm_medium=rss
Health and Safety Executive and Health and Safety Laboratory. (2018). Manual Handling Assessment Charts (the MAC tool). INDG383(rev3).
Karwowski, W. (1988). Maximum load lifting capacity of males and females in teamwork. Proceedings of the Human Factors Society 32nd Annual Meeting.
Marras, W.S., Davis, K.G., Kirking, B.C., Granata, K.P. (1999). Spine loading and trunk kinematics during team lifting. Ergonomics 42, 1258-1273.
Sharp, M.A., Rice, V.J., Nindl, T.L., Williamson, T.L. (1993). Maximum team lifting capacity as a function of team size. Technical Report T94-2. U.S. Army Research Institute of Environmental Medicine.
U.K. Ministry of Defence. (2016). Management of health and safety in defence: arrangements (JSP 375 part 2, volume 1).
U.S. Department of Defense. (2012). MIL-STD-1472G Design Criteria Standard: Human Engineering.
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